Hi
To prevent the code being removed from circuit, we need secure flash when we program it. To secure the flash, we need set Flash Security Register (FSEC).
Please search reference manual with keyword "FSEC"
If you want to secure it with back door key(thus ctm can not see the code but worker still can see the code with backdoor key), the attached document and its related code will be helpful. The example code is for MC9S12DP256, but the method and theory is the same to other HCS12 family MCUs.
http://cache.nxp.com/files/microcontrollers/doc/app_note/AN2880.pdf
software for AN2880
Have a great day,
Jennie Zhang
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